What diameter and length of flexible hose is suitable for me
What diameter and length of flexible hose is suitable for me – a complete guide to choosing the right dimensions
This seemingly simple question – yet in practice it belongs to those where customers most often make a mistake. They buy a hose that is too short and have to go back to the store for it, or choose a diameter that does not fit the existing threads. The result is always the same: unnecessary costs, wasted time, and sometimes even a water leak. In this article, we therefore address the topic of dimensions of flexible hoses in great depth – from basic concepts through specific dimensions for individual applications to practical tips from installation practice.
Why dimensions matter more than you think
Flexible hoses may appear at first glance to be a universal solution – they bend, adapt, and compensate for deviations. And yes, precisely for this reason they are so popular in heating reconstructions, when connecting boilers, valves, distributors, or plumbing equipment. However, their flexibility has physical limits. A hose that is too short will be stretched, excessive mechanical stress arises at the bend points, and the connection is prematurely endangered. On the other hand, a hose that is too long unnecessarily twists, forms loops in which air or impurities accumulate, and in confined spaces it simply gets in the way.
Diameter is an even more sensitive issue. An incorrect internal diameter directly affects the flow – in the case of an undersized hose, pressure loss increases and the system loses performance. An incorrect external diameter of the thread, on the other hand, means that the hose simply does not fit the valve or the sealing does not seat properly. This topic is discussed in more detail in the article Pressure and temperature – what to pay attention to when choosing a flexible hose in our Knowledge Center.
Basic terms: what exactly do the dimensions on a flexible hose mean
Before we get to specific numbers, let's clarify the terminology – in practice, we see that most misunderstandings arise precisely here.
Internal diameter (DN or ID)
The internal diameter of the hose determines how much water flows through it per unit of time. It is marked either as DN (Diameter Nominal – nominal diameter, a rounded value in millimeters) or the abbreviation ID (Inner Diameter – actual internal diameter). In heating flexible hoses, we most commonly encounter values of DN 10, DN 12, DN 15, DN 20, and DN 25. The smaller the DN, the greater the hydraulic resistance for the same flow rate.
External thread diameter (G or Rp)
Flexible hoses have a thread at the ends – either external (G = BSP external) or internal (Rp = BSP internal). Dimensions are given in inches: 1/2", 3/4", 1" are absolutely the most common. Note: the number in inches does not directly correspond to the physical size of the thread in millimeters – it is a historically rooted convention. A 1/2" thread has an actual external diameter of about 20.9 mm, a 3/4" thread has about 26.4 mm. This is why it is not enough to measure the thread with a regular ruler – you need to know what type it is.
Total hose length (L)
The length is usually given in millimeters and is measured including the fittings (total installation length). Sometimes the manufacturer specifies the length of the braid only (without fittings) – in this case, you need to add about 30–50 mm on each side for the fittings. Always check what length the catalog means – total or just the hose itself.
How to measure the correct length of a flexible hose
This is where the most mistakes are made. It is not enough to measure the distance between two valves with a ruler "from end to start". You need to think about several things at once.
Measuring the distance between thread centers
The correct procedure is to measure the distance from the center of one thread to the center of the other thread (from the face of the flange, or from the end of the threaded part, to the same point on the other side). This value corresponds to the installation length of the hose. If the installation space is not straight, and the hose has to go around a bend or through a corner, take a piece of wire or a flexible tape measure, run it along the planned route, and then measure its length – this is your actual installation length.
Reserve for movement and expansion
In heating systems, pipes expand and contract due to temperature. Metal flexible hoses – stainless steel braided – are also designed to absorb these expansions. If the hose is exposed to thermal cycles (boiler, heating circuit), always choose a hose that is 10–15% longer than the actual measurement. So if the distance comes out to 300 mm, take a 350 mm hose. This reserve allows the hose to work in a slightly curved state, which is mechanically more favorable for it than being stretched completely flat.
Space for installation and wrench
Do not forget: you need to be able to fit a wrench – a flat or box wrench – on each fitting. If the thread is in a corner, in a tight distributor, or behind a radiator where there is less space, take a shorter hose and use the space for a bend, or make sure you have thin-walled wrenches in advance. Think about this before ordering, not during installation.
Overview of standard lengths of flexible hoses and their typical use
Flexible hoses are available on the market in a wide range of lengths – from miniature 100 mm (10 cm) up to industrial 2 000 mm and more. In professional heating and standard plumbing installations, we usually work within several key ranges in practice:
| Length (mm) | Typical use | Note from practice |
|---|---|---|
| 100 – 150 mm | Connecting nearby valves, gas valve connection, tight installation in a manifold | Be careful – a too short hose cannot be bent properly and must not be stretched straight |
| 200 – 300 mm | Connecting boiler to manifold, short valve connections, radiator connections | The most universal range, covering most common situations |
| 350 – 500 mm | Connecting pump, boiler to more distant valve, bypassing an obstacle | Suitable where the hose needs to be led in a curve or with a greater deviation |
| 600 – 800 mm | Longer connections, solar collectors, storage heaters, emergency bridging | Pay attention to hose anchoring, a longer hose in a vertical position requires support |
| 1 000 mm and more | Technological connections, hose lines in machine rooms, long-distance bridging | Usually made to order, in the catalog as a special version |
From real practice, I can say that lengths of 250 mm and 400 mm are those I reach for most often. These two values cover 80% of common situations when connecting a boiler, pump, balancing valves or groups of solar panels. If you are unsure and the price difference is not dramatic, always take a hose one step longer – it can be bent, but not stretched.
Diameters of flexible hoses – what thread sizes reveal
The diameter of a flexible hose is directly related to the thread size on its ends. It is not an identical value (1/2" thread ≠ hose with 1/2" internal diameter), but there is an established convention that needs to be known.
Dependence of thread, internal diameter and application
| Thread (inches) | Actual external thread diameter (mm) | Typical hose internal diameter (mm) | Typical device / application |
|---|---|---|---|
| 3/8" | 16,7 mm | 8 – 10 mm | Low-power radiator valves, some valves |
| 1/2" | 20,9 mm | 10 – 13 mm | Radiator connections, taps, boiler valves up to 20 kW |
| 3/4" | 26,4 mm | 16 – 20 mm | Boiler connections, pumps, manifolds, solar collectors |
| 1" | 33,2 mm | 22 – 26 mm | Larger boilers, storage tanks, technological devices |
| 1 1/4" | 41,9 mm | 30 – 35 mm | Industrial applications, large-volume boiler rooms |
In the segment of family homes and apartments, you mostly encounter hoses of 1/2" and 3/4". A diameter of 1" comes into consideration for larger boilers over 35–50 kW or for heat pump collectors. If you are renovating apartment heating, check the existing threads on the boiler and manifold and choose the hose diameter accordingly.
Combined (different) threads on ends – when and why
Not always are both threads on the hose the same. In practice, we often encounter hoses that have an external thread (G – screwed into the valve nut) on one side and an internal thread (Rp – the valve is screwed into the hose) on the other. Or both threads are external or both internal.
Example from practice: when connecting a boiler, where the supply has an external thread G 3/4" sticking out and the manifold has an internal thread Rp 3/4" on the inlet, you need a hose with an internal thread on the boiler side and an external thread on the manifold side – or a classic hose with two internal threads and an adapter on the side where it does not fit. Therefore, always specify the type of thread (external/internal) you need on each side when ordering.
If you are unsure about the type of thread on your valves, the article How to choose a flexible hose for water in a heating system provides a detailed procedure for identifying threads including comparative photos and measuring techniques.
Impact of diameter on the hydraulic performance of the system
This is a topic that many installers underestimate, because "it's just a hose, the boiler determines the performance." Not quite. Every component in the hydraulic circuit – including flexible hoses – contributes to the total pressure loss of the system. And pressure loss directly affects whether the circulation pump can ensure the designed flow, or will operate outside its characteristics.
Pressure loss in a hose increases with the square of the flow velocity, and the velocity depends on the diameter. Simplified: if you halve the hose diameter at the same flow rate, the pressure loss increases roughly sixteenfold. That's a huge difference. So if the boiler or system specifies a 3/4" connection and you install a 1/2" hose to save space, the pump will feel it.
Proper dimensioning: the diameter of the flexible hose should not be smaller than the diameter of the device's connection to which it is attached. If the diameter changes (e.g., the boiler has 3/4" and the manifold has 1"), either use a 3/4" hose with a reducer or a straight hose on the larger side – better to solve the reduction with a valve, not with a hose.
Minimum bend radius – what happens if you kink the hose
Every flexible hose has a defined minimum bend radius (minimum bend radius). This is the minimum distance from the center of the bend at which the hose can still not "crimp" – that is, it does not deform permanently and does not endanger its function. For common braided stainless steel flexible hoses, this radius ranges roughly from 50 to 150 mm, depending on the diameter and construction.
In practice, we see cases where an installer bends the hose almost into a right angle at the thread itself – the braid is partially twisted inward, and after several years of thermal cycles, a leak appears at this point. Never bend the hose directly at the threaded end – leave at least 4–5 cm of straight section before the start of the arc.
If you need a larger angle of deviation and there is not enough space for a smooth arc, it is better to use an angled adapter (elbow) on the valve and then install a shorter hose with a milder arc, rather than forcing a long hose into a sharp bend.
Special cases and situations from practice – what might catch you off guard
Reconstruction behind the radiator
When replacing a radiator or valves in a residential heating system, you often encounter space limitations behind the radiator. The distance from the wall to the radiator connection is usually 50–120 mm. Short hoses of 150–200 mm are usually sufficient here, but you must know exactly what thread the valve has (usually 1/2") and what the radiator connection thread is. Be careful with older radiators with 1"-threads – when replacing them with a modern radiator with a 1/2"-connection, you need a reducer.
Solar system – high temperatures, specific lengths
Solar collectors operate at temperatures of typically 80–120 °C and even higher during stagnation. Here, a standard heating hose is not enough – you need hoses certified for these temperature ranges. Lengths are chosen to compensate for thermal expansion of the piping and to dampen pump vibrations. More about temperature limits can be found in the article Pressure and temperature – what to watch out for when selecting a flexible hose.
Pump – always a hose, never a rigid connection
The circulation pump of the heating system generates vibrations. If the pump is connected to the boiler or manifold with a rigid pipe, these vibrations are transferred throughout the installation – humming, excessive thread wear, leaks. Flexible hoses are an absolute necessity here. The length is chosen so that the hose can work with a slight arc (20–30 degrees) – usually 300–400 mm on each side of the pump. The diameter must match the pump connections, typically 3/4" or 1".
Hot water storage tank – watch both circuits
A hot water storage tank usually has two circuits: primary (heating/boiler) and secondary (distribution of hot water to the residential network). Both circuits may have different thread diameters and require different hoses. The primary circuit of a 300 l+ tank is usually 1", the secondary 3/4". Do not mix these hoses – and if the tank is to be used for potable water, require hoses certified for contact with potable water. This topic is covered in detail in the article What materials of flexible hoses are suitable for potable and service water.
Practical selection process: step by step
To conclude this section, I have summarized the process I use for every job – it works reliably for both experienced installers and for a handy homeowner who wants to take on the work themselves:
- Determine the type of device and the diameter of its connections – check on the label, in the documentation, or measure the thread diameter directly.
- Measure the mounting length (from the center of the thread to the center of the thread, along the actual hose route).
- Add 10–15% extra for expansion and bending – round up to the nearest catalog size.
- Check the type of thread on each side (external G / internal Rp) and select the hose ends accordingly.
- Verify the temperature and pressure limits of the hose against the parameters of your installation – for a boiler circuit, 120 °C / 10 bar is usually sufficient, for solar systems, you need more.
- If the hose will be in contact with potable water, verify the material certification.
- When ordering multiple pieces (e.g., for an entire boiler room), order one extra piece – a spare is useful sooner than you expect.
More details on installation can be found in the article Installation of a flexible hose for water step by step, where the process is supplemented with instructions for tightening connections, selecting gaskets, and checking for leaks.
Most common mistakes in selecting dimensions – what we see all around
Over the years of practice, certain mistakes repeat so regularly that it's a shame not to mention them. Do you recognize them? If yes, you know what to avoid. If not – here's your chance to avoid repeating the mistakes of others.
- Measuring distance in a straight line – the hose will not go straight, always measure along the route.
- Forgetting about the fittings – does the catalog length include or exclude the fittings? Always check.
- Assuming that a 1/2" thread = 1/2" hose diameter – it's not, see the table above.
- Selecting an overly thin hose to save money – at higher flow rates, this generates unnecessary pressure losses.
- Mounting the hose stretched completely straight without any arc – this is the fastest way to a leak during expansion.
- Mixing up external and internal threads – a mismatched thread cannot be forced in, even with force, hot water, or additional tape.
- Ignoring the minimum bend radius – a bent hose at the thread is a silent threat, not an immediate problem.
The causes of leaks and malfunctions are also discussed in detail in the article Common leaks and malfunctions of flexible hoses – causes and solutions, where you will also find diagnostic tips for identifying the problematic location.
Quick selection table – diameter and length according to device
| Device / application | Thread | Recommended length | Note |
|---|---|---|---|
| Panel radiator (small, up to 1 500 W) | 1/2" | 150 – 250 mm | Both ends with the same thread, check valve type |
| Condensing boiler up to 25 kW | 3/4" | 300 – 500 mm | Usually 2 pcs (supply/return), check boiler manual |
| Condensing boiler 25 – 50 kW | 1" | 400 – 600 mm | Larger diameter = lower losses, always according to boiler connection |
| Circulation pump (vibrations!) | 3/4" – 1" | 300 – 400 mm (each side) | Mild bend required – vibration damping |
| TÚV tank – primary circuit | 3/4" – 1" | 350 – 500 mm | According to tank volume and heat source power |
| TÚV tank – secondary circuit (drinking water) | 3/4" | 200 – 400 mm | Material certification for drinking water is required! |
| Solar collector | 3/4" – 1" | 300 – 600 mm | Temperature resistance min. 150 °C, certification for solar medium |
| Heat pump – indoor unit | 1" – 1 1/4" | 400 – 800 mm | Always according to the project and manufacturer documentation of the heat pump |
Frequently asked questions (FAQ)
Can I use a longer hose than necessary – for example, 500 mm instead of 300 mm?
Yes, a longer hose is usually better than a shorter one – provided that the excess length can be bent into a mild curve and the hose does not interfere with or touch surrounding objects. Never route the hose around a corner or through a space in a way that would create a sharp bend or unnecessary pressure stress. If you cannot store it properly, it is better to choose the correct length.
What if I cannot measure the length accurately before installation, because the old piping is not yet removed?
In such a case, you can estimate based on the project documentation or schematic, or by measuring a similar installation in adjacent apartments. Alternative: order hoses one step longer (e.g., 400 mm instead of 300 mm) and save them by bending into a curve during installation. For precise measuring procedures, read the article Installation of flexible water hoses step by step.
What is the difference between a hose with G 1/2" and Rp 1/2"?
G 1/2" denotes an external thread (screwed into a nut or valve body), while Rp 1/2" is an internal conical thread according to the BSP standard (the hose body accepts the external thread of the valve). In practice: if the valve has an external protruding thread, you need Rp (internal) on the hose. If the valve has a threaded hole, you need G (external) on the hose. Always check both types before ordering – they may differ on each end of the hose.
Can I connect a 3/4" hose to a 1/2" valve without an adapter?
No. Threads of different sizes cannot be connected directly – different pitch, different diameter, the sealing will not hold. Always use a reducer or adapter (e.g., 3/4" × 1/2"), which is a valve with two different threads. Trying to connect them directly will damage the thread and the connection will leak.
How long does a stainless steel flexible hose last in heating systems?
A high-quality stainless steel braided flexible hose, when properly installed and within its temperature and pressure limits, typically lasts 15–25 years. It is critical that it is not stressed by bending, vibrations without damping, and that the sealing is regularly checked – usually during the annual boiler service. More tips can be found in the article How to extend the lifespan of flexible hoses in heating and plumbing systems.
Do I have to replace all flexible hoses during reconstruction, or only the damaged ones?
It depends on their condition and age. If the hoses are older than 15 years and show signs of corrosion, deformation of the braid, noticeable discoloration, or any signs of leakage, I recommend replacing all of them at once – it will save you from repeated draining of the system. If the hoses are visually in good condition and younger, it is sufficient to replace the problematic ones. The topic of reconstruction is discussed in more detail in the article Flexible hoses during heating system reconstruction – what you need to know in advance.
Conclusion: the right size = peaceful sleep
Selecting the correct diameter and length of a flexible hose is not rocket science – but it is not a task you should solve "by eye" or based on what is in stock. Every incorrectly chosen hose will eventually cost you: repairs, dry floors, pressure drop in the system, or re-digging for a manifold.
The key is to measure correctly, allow for some reserve, identify thread types, and choose a hose that meets the real conditions – not only length and diameter, but also temperature, pressure, and medium. If you are unsure about anything, check other articles in the Knowledge Center – How to choose a flexible hose for a heating system, Braided vs. smooth flexible hoses for heating – comparison, or Common questions about flexible water hoses – where you will find answers to further technical details.
And if you need advice for a specific project, the full range of flexible water hoses is clearly listed on atria.sk in the category of flexible water hoses – including filtering by diameter, length, and thread type.
Do you have a question about this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.